Multi-channel radiometer imaging system

ABSTRACT

A radiometer system includes a housing and an RF board contained within the housing. Balanced radiometer channels receive an unknown signal and known reference. Each balanced radiometer channel includes at least one quadrature hybrid and amplifier circuit to provide distributed gain and an amplified output signal. A detection circuit receives and detects the amplified output signal forming a detected signal. A controller board is contained within the housing and has an integration circuit and microcontroller that receives the detected signal and performs video signal digitization and conditioning and real-time corrections on any radiometer channels to account for changes in temperature or gain.

RELATED APPLICATIONS

This application is based upon prior filed provisional application Ser.No. 60/504,182 filed Sep. 18, 2003 now abandoned.

FIELD OF THE INVENTION

The present invention relates to the field of focal plane radiometers,and more particularly, the present invention relates to a multi-channelradiometer applicable for use at millimeter wave (MMW) frequencies.

BACKGROUND OF THE INVENTION

Since radio waves may be considered infrared radiation of long wave, ahot body would be expected to radiate microwave energy thermally. Inorder to be a good radiator of microwave energy, a body must be a goodabsorber. The best thermal radiator is a “black body.” The amount ofradiation emitted in the MMW range is 10⁸ times smaller than the amountemitted in the infrared range. Current MMW receivers, however, have atleast 10⁵ times better noise performance than infrared detectors, andwith some temperature contrast, the remaining 10 ³ may be recovered.This makes passive MMW imaging comparable in performance with currentinfrared systems. This unique characteristic makes MMW radiometers apopular choice for sensing thermal radiation. MMW radiometers have beenused in many different applications such as remote terrestrial andextra-terrestrial sensing, medical diagnostics and defense applications.MMW electromagnetic radiation windows occur at 35 GHz, 94 GHz, 140 GHzand 220 GHz. The choice of frequency depends on specific applications.

Focal plane arrays are used to form images from radiation received by areflector antenna. Millimeter wave (MMW) focal plane array radiometersalso have been used in many applications to form images based on thermalsensing of radiated microwave energy. The sensitivity of existingradiometer designs, however, has been limited to about 1 deg K,resulting in poor images.

The principle of operation of the radiometric technique is fullydescribed in the literature. The design of a typical radiometer is basedon the technique of comparing the level of electromagnetic noise emittedby an unknown source to a reference or stable noise source. Thistechnique and devices were initially proposed by Dicke [R. H. Dicke,“The Measurement of Thermal Radiation at Microwave Frequencies,” TheReview of Scientific Instruments, Vol. 17, No. 7, July 1946].

In a Dicke radiometer circuit, the signals from an antenna are sampledand compared with signals from a reference source maintained at a knownconstant temperature. This overcomes some of the problems of amplifierinstability, but in general does not alter effects resulting fromimperfect components and thermal gradients.

While other types of radiometric devices have been used with somesuccess, the Dicke (or comparison) type of radiometer has been the mostwidely used for the study of relatively low level noise-like MMWsignals, especially where the noise signals to be examined are oftensmall in comparison to the internally generated noise level within theradiometer receiver. While there are several types of comparisonradiometers, one popular type of radiometer for use in themicrowave/millimeter wave frequency bands is that in which an incomingsignal to be measured and a standard or calibrated reference noisesignal are compared. This type of radiometer consists essentially of thecomparison of the amplitude of an unknown noise signal coming from thesource to be examined with a known amplitude of a noise signal from acalibration source. This method has been found useful in measuring withconsiderable accuracy the effective temperature of an unknown source.

In the Dicke or comparison type radiometer, the receiver input isswitched between the antenna and a local reference signal noisegenerator. The detected and amplified receiver output is coupled to aphase-sensing detector operated in synchronism with the input switching.The output signal from such a radiometer receiver is proportionate tothe difference between the temperature of the reference signal sourceand the temperature of the source viewed by the antenna inasmuch as thephase-sensing detector acts to subtract the background or internal noiseof the receiver.

A Dicke radiometer uses an RF switch coupled between an antenna and aradiometer receiver, allowing the receiver to alternate between theantenna and a known reference load termination. The receiver output isconnected to a synchronous detector that produces an output voltageproportional to a difference between the antenna and the referencetemperature. Null balance operation for the Dicke radiometer has beenachieved by coupling in noise from a hot noise diode to the antenna portof the RF switch thereby enabling matching the temperature from standardreference loads.

The sensitivity of radiometer measurements are also often limited byrandom gain fluctuations in the RF front end, low frequency noise (1/f),and bias in the detector circuits. Over the last decades many specialtechniques, including Dicke switching, have been implemented to reducemeasurement errors. Many of these proposals do not yield a true solutionthat will allow MMW radiometers to be commercially viable. In addition,the high cost of MMW RF receivers has limited the number of channels inthe radiometer to a low number, resulting in a requirement to scan bothazimuth and elevation to create an image.

SUMMARY OF THE INVENTION

The present invention eliminates the need for a Dicke switch and doesnot use a synchronizing circuit because it uses the source and referenceall the time, and runs the source and reference signal through theamplifiers. The present invention uses a balanced channel approach usingpreferred MMIC chips. Thus, a radiometer channel can be implemented bythe use of either a single millimeter wave monolithic integrated circuit(MMIC) or through discrete implementation using printed hybrids andmultiple MMIC low noise amplifiers (LNA's).

The radiometer module of the present invention has at least six timeshigher sensitivity than current radiometer sensitivity, such as a Dickeradiometer sensitivity. The present invention is advantageous andprovides a radiometer with the sensitivity of less than 1° K. Theradiometer module (sensor) is at least ten times smaller than otherradiometers currently in use, which typically measure more than threecubic feet even with a small number of channels. The radiometer of thepresent invention also is at least ten times lighter than existingradiometers, which weigh no less than 20 pounds even with a small numberof channels. The radiometer of the present invention is less than aboutthree pounds.

The present invention also is self-correcting for temperature and gainvariations. It uses a balanced pair of diodes for detection and thechopper operational amplifiers to eliminate any bias and reduce 1/Fnoise. The microcontroller can monitor temperature changes between theantenna and the reference by reading any temperature sensors located onthe antenna and near the reference. This can be based on temperatures toadjust the correction factor. The gain can be continuously monitored andthe bias adjusted of the low noise amplifier (LNA) to maintain theconstant gain. Real-time corrections can be performed on all videochannels to account for any changes in temperature or gain.

The radiometer of the present invention also has self-healing capabilitybecause of the distributed gain approach. Failure of one or more LNA'sin each channel will not result in failure of the channel. Themicrocontroller can compensate for the drop of any amplifiers in thechain.

The compact radiometer of the present invention can fit directly in theantenna focal plane. A quadrature hybrid network is used in the frontend to distribute RF input signals and reference signals to a balancedamplifier chain, thereby reducing gain variations and improvingradiometer sensitivity. A balanced detector diode circuit, for example,a pair of diodes in one non-limiting example, eliminates drift errorsintroduced by a detector diode as a function of temperature.

A video signal chopper amplifier circuit, also referred to by some as aauto zero amplifier, eliminates bias introduced by the video amplifier.A near perfect channel-to-channel matching exists through the use ofquadrature hybrid network or through digital signal processingcorrections.

The radiometer system of the present invention includes a housing and anRF board contained within the housing. Balanced radiometer channelsreceive an unknown signal and known reference. Each balanced radiometerchannel includes at least one quadrature hybrid and amplifier circuit toprovide distributed gain and an amplified output signal. A detectioncircuit receives and detects the amplified output signal forming adetected signal. A controller board is contained within the housing andhas an integration circuit and microcontroller that receives thedetected signal and performs video signal digitization and conditioningand real-time corrections on any radiometer channels to account forchanges in temperature or gain.

In accordance with another aspect of the present invention, a quadraturehybrid and amplifier circuit comprises a MMIC chip. It can also beformed as discrete printed hybrids and low noise amplifiers. The RFboard is preferably formed from a soft board or ceramic material. Aradiometer system of the present invention is sized and of such weightto fit directly into an antenna focal plane. It includes a cover havingRF launch openings that are typically sized to allow for one wavelengthspacing between any sensing radiators

A video signal chopper amplifier circuit can be contained on thecontroller board to receive the detected signal and aid in eliminatingbias and reducing 1/f noise. The detection circuit can include at leastone pair of balanced diodes. The controller board is also adapted tointerface with an external display system. A plurality of sensingradiators are formed on the RF board. The analog-to-digital conversioncircuit is positioned on the controller board and digitizes detectedsignals received from the integration circuit for processing by themicrocontroller. Isolation vias can be formed on the RF board andisolate the quadrature hybrid and amplifier circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome apparent from the detailed description of the invention whichfollows, when considered in light of the accompanying drawings in which:

FIG. 1 is a fragmentary environmental view of a typical radiometerantenna system for a focal plane array.

FIG. 2 is a block diagram showing a receiver front end in a typicalradiometer system.

FIG. 2A is a block diagram showing how radimeter modules are typicallyconnected to the antenna with a waveguide manifold in current art.

FIG. 3 is a block diagram illustrating the basic functional componentsof the radiometer of the present invention.

FIG. 3A is a block diagram of the quadrature hybrid used in theradiometer of FIG. 3 showing how inputs A, B are divided equally in thefirst hybrid, then reconstructed in the second hybrid.

FIG. 3B is a block diagram showing a two-stage MMIC LNA chip of thepresent invention as a representative example.

FIG. 3C is a block diagram showing a three stage MMIC LNA chip of thepresent invention.

FIG. 3D is a block diagram illustrating the basic functional componentsof the radiometer of the present invention using the MMIC chip of FIG.3B.

FIG. 4 is a block diagram showing functional components of anotherexample of a multi-channel radiometer of the present invention.

FIG. 5 is a block diagram showing the layout for the RF front end in theradiometer of the present invention.

FIG. 6 is a plan view showing a multi-channel radiometer layout on asingle RF board.

FIG. 7 is an exploded isometric view of a compact multi-channelradiometer module showing a base housing, RF board, controller board andtop cover.

FIG. 8 is an isometric view of the assembled multi-channel radiometermodule of the present invention.

FIG. 9 is a top plan view of the multi-channel millimeter waveradiometer module shown in FIG. 8.

FIG. 10 is a chart showing Dicke radiometer sensitivity of the typeshown in FIG. 2.

FIG. 11 is a chart showing the radiometer sensitivity of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout, and prime notation is used toindicate similar elements in alternative embodiments.

The present invention overcomes many existing shortcomings of currentradiometers, including gain variation of the amplifiers, low frequencynoise, detector bias, low sensitivity and high cost. The presentinvention reduces cost and size of a radiometer by a least a factor often and provides a commercial advantage over many current radiometers.

The low cost MMW radiometer of the present invention includes a housingsection, a multi-channel RF board, and a controller board. The housingsection is preferably made up of a base metal housing and a metallizedplastic or metal cover that includes the RF launch opening, typicallyfilled with a dielectric material, for example, a plastic material toallow for less than one wavelength spacing between the sensingradiators. The RF board preferably is formed from a single soft board orceramic material. All MMW microstrip circuits, for example, 50 Ohmlines, filters, 90° hybrids and RF radiators, are printed on this board.MMIC amplifiers can be either attached directly to the board or, throughcut-outs, on a carrier plate underneath to the RF board. A simple MMICchip can also be used. A controller board, which uses a low costmicrocontroller, performs any necessary video signal amplification,digitization and conditioning, automatic RF amplifier bias adjustment,and DC power regulation. The controller board interfaces with anexternal display system.

The compact radiometer of the present invention can fit directly in theantenna focal plane. A quadrature hybrid network is used in the frontend to distribute RF input signals and reference signals to a balancedamplifier chain, thereby reducing gain variations and improvingradiometer sensitivity. A balanced detector diode circuit, for example,a pair of diodes in one non-limiting example, eliminates drift errorsintroduced by a detector diode as a function of temperature.

A video signal chopper amplifier circuit, also referred to by some as aauto zero amplifier, eliminates bias introduced by the video amplifier.A near perfect channel-to-channel matching exists through the use ofquadrature hybrid network or through digital signal processingcorrections.

FIG. 1 shows a typical radiometer antenna system 20. The main antenna 22collects temperature data or other pertinent data to be analyzed. Thedata is focused in the middle of the antenna at the focal plane array 24using a sub-reflector 26.

FIG. 2 shows a common prior art “Dicke” type radiometer system 30,including a receiver front end. In a Dicke radiometer, generally areceiving circuit detects weak signals in noise and modulates thesesignals at an input. The circuit demodulates the signals and comparesthe output with a reference from the modulator. Coincidence indicates asignal presence. For example, microwave noise power can be measured bycomparing it with the noise from a standard source in a waveguide.

In this illustrated example of a Dicke radiometer, the antenna 32 sensestarget temperature, which is proportional to the radiated target energy.The energy passes through a Dicke switch 34 of the type known to thoseskilled in the art and into a series of MMIC amplifiers 36 a, 36 b, 36c. A band pass filter 38 sets the receiver bandwidth. A square lawdetector 40 detects the signal and passes it to an integrator 42, whichsums the signal over an observation period. A data acquisition andprocessing circuit 44 receives the integrated signal, where it isdigitized, compensated for gain variation, and processed for display ona video or for further processing. To cancel the effects of gainvariation, the Dicke switch 34 samples a reference source 46. Gainvariations in the receiver are cancelled using the measured referencegain.

Radiometer sensitivity is important. The precision in estimating themeasured temperature is often referred to as the radiometer sensitivity,ΔT. This parameter is a key quantity characterizing the performance of aMMW radiometer. In radiometer terminology, this is the smallest changein temperature that can be detected by the radiometer. The equation,which derives the sensitivity of the system 30 shown in FIG. 2 is:P _(sys) =P _(A) +P _(rec)where P_(sys)=total input power

-   P_(A)=Noise power at the antenna=k T_(A) B-   P_(rec)=Noise power generated in the receiver=kT_(rec)B-   K=Boltzmann's constant-   B=receiver bandwidth

Assuming a square law detector, the radiometer output voltage is anaverage value of the radiometer output noise power. The square lawdetector can have an output proportional to the square of the appliedvoltage, e.g., the output is proportional to the square of the inputamplitude. A radiometer output voltage is:V _(out) =P _(sys) ×G _(sys)where G_(sys) is the receiver gain.

Assuming that G_(sys) and T_(rec) are constant, the radiometersensitivity is:ΔT _(ideal)=(1/√{square root over (Bτ)})T _(sys)where τ is the integration time.

In most applications, however, G_(sys) and T_(rec) are not constant, andtheir variations cause degradation of the radiometer sensitivity asfollows:

Gain variations effects:ΔT _(G)=(T _(A) −T _(ref))×(ΔG _(sys) /G _(sys))

Assuming a five degree difference between the antenna temperature andthe reference temperature, a +/−3 dB gain variation (over the 3 LNA's 36a, 36 b, 36 c), and a 40 dB total system gain, the radiometersensitivity will vary by about 5%.

Temperature variation effects can be shown:ΔT _(ant)=(T _(A) +T _(rec))/(√{square root over (Bτ/2)})=√{square rootover (2)}(T _(A) +T _(rec))/(√{square root over (Bτ)})ΔT _(ref)=(T _(ref) +T _(rec))/(√{square root over (Bτ/2)})=√{squareroot over (2)}(T _(ref) +T _(rec))/(√{square root over (Bτ)})Assuming statistical independence, the temperature variation can beshown:

$\begin{matrix}{{\Delta\; T} = \left\lbrack {\left( {\Delta\; T_{G}} \right)^{2} + \left( {\Delta\; T_{ant}} \right)^{2} + \left( {\Delta\; T_{ref}} \right)^{2}} \right\rbrack^{1/2}} \\{= \left\lbrack {\frac{{2\left( {T_{A} + T_{rec}} \right)^{2}} + {2\left( {T_{ref} + T_{rec}} \right)^{2}}}{\left( B_{\tau} \right)^{1/2}} + {\left( {\Delta\;{G_{sys}/G_{sys}}} \right)^{2}\left( {T_{A} - T_{ref}} \right)^{2}}} \right\rbrack^{1/2}}\end{matrix}$Assuming a balanced Dicke radiometer (i.e. T_(A)=T_(ref)), the aboveequation can be simplified to:

$\begin{matrix}{{\Delta\; T} = {2{\left( {T_{A} + T_{rec}} \right)/\sqrt{{B\;}_{\tau}}}}} \\{= {2\mspace{14mu}\Delta\; T_{ideal}}}\end{matrix}$

Therefore, the Dicke radiometer sensitivity is twice that of an idealtotal power radiometer. The factor of two (2) comes about because theDicke switch alternates between the reference and the antenna such thatT_(A) is observed for only half of the time.

FIG. 2A shows how the radiometer channels 48, indicated as channels 1 .. . N, as part of RF modules, are typically connected to the antenna 48a. Because of the large size of the radiometer RF modules, which cannotfit directly in the antenna focal plane, a waveguide manifold 48 b isused to connect the modules to the focal plane. The waveguide manifold48 b increases the front end losses by at least 2 dB, resulting inreduced radiometer sensitivity. The channels 48 connect to dataacquisition and processing circuit 48 c.

FIG. 3 is a block diagram of the radiometer 50 of the present invention.This radiometer design does not use a Dicke switch, yet it stilldelivers superior sensitivity and can be readily manufactured.

A radiator 52 provides a first signal input A while a reference 54provides a second signal input B. The radiator 52 could be many types ofradiator elements used in radiometrs, including an antenna. Microstripquadrature hybrid circuits 56 are operable with low noise amplifiercircuits 58. The hybrid circuits can be 90° hybrids. Bandpass filtercircuits 60 a, 60 b receive the signals represented at A and B, whichare output to detector circuits 62 a, 62 b. These components aretypically mounted on an RF board indicated by the dashed lines at 64.The RF board is typically formed from a single soft board or ceramicmaterial. All MMW microstrip circuits, for example, 50 ohm lines,filters, hybrids and RF radiators, are printed on this board. Any MMICamplifiers can be attached directly to the board, or through cut-outs,on a carrier plate underneath to the RF board.

The signals (A and B) are output to a controller board indicated bydashed lines at 70. On this board, any necessary video signalamplification, digitization and conditioning, automatic RF amplifierbias adjustment, and DC power regulation occurs. This board caninterface directly with a video display system. The signal is receivedat two chopper amplifier circuits 72 a, 72 b. After amplification, thesignals are integrated at integrator circuits 74 a, 74 b, and digitizedat analog/digital (A/D) circuits 76 a, 76 b. A microcontroller circuit78 provides digital video processing and receives an antenna temperaturesignal 80, amplifier control signal 82, and reference temperature signal84. The output from the microcontroller circuit 78 is sent to a displayor other external sensors 86.

The radiometer 50 of the present invention uses microstrip quadraturehybrids 56 to distribute the signal and reference powers to the balancedamplifier chain as illustrated. The pairs of low noise amplifiers(LNA's) 58 are cross-coupled to each other, similar to a conventionalbalanced amplifier configuration.

The quadrature hybrid shown in FIG. 3A is a well known four-port devicethat splits the energy into equal parts at the output, but with a 90degree phase difference. For example, the signal A at the input port P1of the hybrid is divided up to two parts at the output ports P3 and P4.The same is true for the input signal B at the input port P2 of thehybrid, which is also divided equally at the output ports P3 and P4.When the output of the first hybrid is used as input into a secondhybrid, the signals A and B are restored at the output of the secondhybrid (of course with some losses due to the hybrids). The two inputs Aand B, which can represent the antenna port and the reference port, orrepresent two antenna ports representing two different polarizations,are divided equally among the amplifiers and reconstructed at theoutput, as shown in FIG. 3A. One other unique feature of this hybriddesign is that failure of one or more of the LNA's 58 in the chain doesnot result in failure of the channel itself. Because of the distributedgain approach, the gain of the channel will drop by a small amount,which can be accounted for in the microcontroller 78. This is differentfrom the traditional radiometer shown in FIG. 2, where failure of oneLNA will result in total failure of the element.

Because each signal passes through each amplifier in the chain, anyfluctuation in the gain of any of the amplifiers is applied equally toboth signals (T_(A) & T_(Ref)). Assuming that the hybrid circuits 56 arewell balanced by using good design practices, this radiometer designguarantees that the gain in each channel is substantially the same. Inaddition, because the gain in each channel is essentially the average ofthat of all the amplifiers in the chain, the overall gain fluctuation iseffectively reduced by a factor of the square root of N, where N is thenumber of amplifiers.

${\Delta\; G_{sys}} = \left\lbrack {\left( {1/N} \right){\sum\limits_{I = 1}^{N}\;\left( {\Delta\; G_{i}} \right)^{2}}} \right\rbrack^{1/2}$

Assuming the same amount of the LNA's gain variation (+/−3 dB) used forthe Dicke radiometer as shown in FIG. 2, the radiometer system gainvariation of the present invention will be only about +/−0.7 dB.Therefore, it is evident that the radiometer of the present inventionprovides the inherent benefits of receiver gain fluctuation reductionand guarantees equal gain for both the antenna power and the referencesignal. This feature provides the same benefit as the Dicke switchwithout the added losses and the complex switching circuitry. Also, theabsence the Dicke switch in the present invention allows continuousobservation of the antenna temperature, thereby achieving thesensitivity of a total power radiometer.ΔT _(ideal)=(1/√{square root over (Bτ)})T _(sys)

Using commercially available W-band LNA's with over 20 GHz bandwidth,such as an ALH394 circuit made by Velocium of Redondo Beach, Calif., andassuming an integration time of 20 msec and 1200 K total systemtemperature, this radiometer sensitivity is less than 0.1 degree. TheALH394 is a broadband, three-stage, low noise monolithic HEMT amplifier.It has a small die size and is passivated. Bond pad and backsidemetallization can be Ti/Au and compatible with conventional die attach,thermocompression and thermosonic wire bonding assembly. It can have ausable radio frequency of 76 to about 96 GHz, linear gain of about 17dB, and a noise figure of about 5 dB depending on applications. It canuse DC power of about 2 volts at 34 mA. Bond pads can include VG1, VG2and VG3, VD1, VD2, VD3, with an RF in and RF out pad.

The RF signals at the output of the band pass filter 60 a, 60 b aredetected using the square law detector 62 a, 62 b. In order to eliminateany detector variation over temperature, a pair of balanced diodes 62 a,62 b, such as a DBES105a diode manufactured by United MonolithicSemiconductors, can be used. This dual Schottky diode is based on a lowcost 1 μm stepper process with bump technology and reduced parasiticconductances and having a high operating frequency. It can be aflip-chip dual diode with high cut-off frequencies of about 3 THz and abreakdown voltage of less than −5 volts at 20 uA. It has a substantiallyadequate ideality factor of about 1.2.

The diodes output an equal amount of power, but with opposite polarity.This method effectively cancels any bias or drift caused by the diodes.The very small DC voltages at the output of the diodes are typicallyvery difficult to amplify accurately. DC offsets introduced by theop-amps are usually a cause of the problem, aggravated often by lowfrequency noise (1/f). The radiometer 50 of the present invention useschopping op-amp circuits 72 a, 72 b, also known as auto zero amplifiers,such as the AD8628 amplifier manufactured by Analog Devices. Thisamplifier circuit eliminates DC offset and low frequency (1/f) noise.

The AD8628 amplifier has ultra-low offset, drift and bias current. It isa wide bandwidth auto-zero amplifier featuring rail-to-rail input andoutput swings and low noise. Operation is specified from 2.7 to 5 voltssingle supply (1.35V to 2.5V dual supply). It has low cost with highaccuracy and low noise and external capacitors are not required. Itreduces the digital switching noise found in most chopper stabilizedamplifiers, and has an offset voltage of 1 μV, a drift less than 0.005μV/° C., and noise of 0.5 uV P—P (0 Hz to 10 Hz). This amplifier isavailable in a tiny SOT23 and 8-pin narrow SOIC plastic packages.

An offset voltage of less than 1 μV allows this amplifier to beconfigured for high gains without risk of excessive output voltageerrors. The small temperature drift of 2 nV/° C. ensures a minimum ofoffset voltage error over its entire temperature range of −40° C. to+125° C. It has high precision through auto-zeroing and chopping. Thisamplifier uses both auto-zeroing and chopping in a ping-pong arrangementto obtain lower low frequency noise and lower energy at the chopping andauto-zeroing frequencies. This maximizes the signal-to-noise radio (SNR)without additional filtering. The clock frequency of 15 kHz simplifiesfilter requirements for a wide, useful, noise-free bandwidth. Theamplifier is preferably packaged in a 5-lead TSOT-23 package.

l/f noise, also known as pink noise, is a major contributor of errors indecoupled measurements. This l/f noise error term can be in the range ofseveral μV or more, and when amplified with the closed-loop gain of thecircuit, can show up as a large output offset. l/f noise is eliminatedinternally. l/f noise appears as a slowly varying offset to inputs.Auto-zeroing corrects any DC or low frequency offset, thus the l/f noisecomponent is essentially removed leaving the amplifier free of l/fnoise.

The output of the integrator circuits 74 a, 74 b for both the antennasignal and the reference signals are digitized using highly linear A/Dcircuits 76 a, 76 b and are sent to the microcontroller 78, where thereference signal is subtracted from the antenna signal to obtain theactual target temperature. The microcontroller 78 can monitor thetemperature of the antenna through a sensor attached to the antenna. Anydifferences between the antenna and the reference are accounted for andcorrections are applied appropriately in software. The microcontroller78 also controls the LNA bias and monitors the amount of current drawnby each amplifier and adjusts the amplifier gain.

FIG. 3B shows a two-stage MMIC chip 112 that can be used in the presentinvention to replace the discrete implementation of the hybrid andcascade LNA's shown in FIG. 3. This MMIC LNA chip receives a signal fromthe antenna 114 or reference load 116 that enters through signal inputsA and B into the hybrid circuit 118 and into amplifiers 120 a, 120 bthrough amplifiers 124 a, 124 b, through hybrid 122 to be output assignals A and B amplified.

FIG. 3C is a block diagram showing a three-stage MMIC LNA chipimplementation 125 with respective amplifier circuits 128 a and 128 b.

Thus, the balanced channel approach of the present invention can useMMIC chips and the implementation of a radiometer channel can occureither by the use of a single millimeter wave monolithic integratedcircuit (MMIC) or through discrete implementation using printed hybridsand multiple MMIC LNA's.

FIG. 3D is a block diagram of the radiometer 50 of the presentinvention. This radiometer design uses the MMIC chip 112 shown in FIG.3B to replace the printed hybrids and individual LNA chips. This figureshows yet another example of how this invention can be used to buildradiometer channels. Only one detector 62 as a pair of diodes is used.One chopper 72 amplifier and any integrator 74 and A/D circuit 76.

The radiometer of the present invention can also be manufactured in anarrangement having a larger number of channels, such as shown in FIG. 4.Prime notation is used to show the various radiators 52′, hybridcircuits 56′, and low noise amplifiers 58′. As illustrated, signals A, Band C are generated from radiators 52′ and a reference signal isgenerated from the reference 54′. Two parallel hybrid circuits 56′ areillustrated at the front end and input into four parallel, low noiseamplifier circuits 58′ instead of two as shown in FIG. 3. This followsby other parallel hybrid circuits 56′ and low noise amplifier circuits58′. Four bandpass filters 60′ are illustrated with detector circuit 62′forming a three-element radio frequency module.

FIG. 5 shows an example of a layout for the RF front end used in theradiometer 50 of the present invention, forming a radiometer cell 90.The radiator elements 52, the quadrature hybrids 56, 50 ohm microstriplines 59 and the filters 60 a, 60 b are all printed on a soft board or aceramic board. Isolation vias 100 are used to isolate the amplifiers 58and reduces the likelihood of oscillations.

FIG. 6 shows a multi-channel radiometer layout on a single RF board. Aplurality of radiometer cells 90 are illustrated, forming a N elementarray 102 with channels 110. Radiators 52 are also illustrated. Thisdesign approach allows for low cost implementation of a large number ofchannels. The radiator elements 52 can be spaced half a wavelength (λ/2)apart for lower cross coupling, lower sidelobes and overall improvedoperations. The channels 110 are stacked on both sides of the board inorder to achieve two rows 110 a, 10 b of radiometer cells 90 in a verysmall amount of space. For dual polarization applications, one row 110 amay be vertically polarized while the second row 10 b could behorizontally polarized. The radiators, for example as antenna elements,can be alternated between vertical and horizontal polarization in thesame row. For example, a 32×2 array can easily fit a 3×4 inch RF board.This board can become part of a radiometer module of the presentinvention.

FIG. 7 shows an exploded view of a compact, multi-channel radiometermodule 130 of the present invention. A base housing 131, typicallymade-up of aluminum, is used to receive the RF board 64, which can beattached to a CTE matched carrier 132. The controller board 70, whichsupplies all the DC voltages and control signals, makes contact with theRF board 64 through the use of DC contact connectors. The top cover 134,which can be made from a plastic material, is metallized everywhereexcept where the radiator areas 136 correspond to the location of theradiators. These unmetallized radiator areas 136 provide a dielectricmedia for the RF energy to travel through. Thus, RF launch openings areformed. A slot 70 a in the controller board provides access to theantenna elements. The entire unit is assembled using fasteners, such asscrews received in fastener apertures 138 a.

FIG. 8 shows fully assembled multichannel radiometer module of thepresent invention forming a radiometer module or “sensor package” asillustrated.

FIG. 9 is a top plan view of the multichannel millimeter wave radiometermodule shown in FIG. 8 and showing the radiator areas 136 and radiators.FIGS. 10 and 11 show various components of a Dicke radiometer (FIG. 10)and showing the Dicke radiometer sensitivity as compared to theradiometer sensitivity of the present invention as shown in FIG. 11. Thedifferent components of the radiometers and the relative sensitivity andoperating or reference values are shown under the specific elements asillustrated.

The radiometer module of the present invention has at least six timeshigher sensitivity than more current radiometer sensitivity, such as theDicke radiometer sensitivity explained above with reference to FIG. 2and shown in FIG. 10. The present invention is advantageous and providesa radiometer with the sensitivity of less than 1° K. The radiometermodule (sensor) is at least ten times smaller than other radiometerscurrently in use. The radiometer of the present invention also is atleast ten times lighter in weight than any other radiometer inexistence, which typically weighs no less than 20 pounds. The radiometerof the present invention is typically less than about three pounds.

The present invention also is self-correcting for temperature and gainvariations. It uses the balanced pair of diodes for detection and thechopper operational amplifiers to eliminate any bias and reduce 1/Fnoise. The microcontroller can monitor temperature changes between theantenna and the reference by reading any temperature sensors located onthe antenna and near the reference. This can be based on temperatures toadjust the correction factor. The gain can be continuously monitored andthe bias adjusted of the low noise amplifier (LNA) to maintain theconstant gain. Real-time corrections can be performed on all videochannels to account for any changes in temperature or gain.

The radiometer of the present invention also has self-healing capabilitybecause of the distributed gain approach. Failure of one or more LNA'sin each channel will not result in failure of the channel. Themicrocontroller can compensate for the drop of any amplifiers in thechain.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

1. A radiometer system comprising: a housing; an RF board containedwithin the housing and including a plurality of balanced radiometerchannels that receive an unknown signal and known reference, eachbalanced radiometer channel having at least one quadrature hybrid andamplifier circuit to provide distributed gain and an amplified outputsignal and a detection circuit that receives and detects the amplifiedoutput signal forming a detected signal; and a controller boardcontained within the housing and having an integration circuit andmicrocontroller that receives the detected signal and performs videosignal digitization and conditioning and real-time corrections on anyradiometer channels to account for changes in temperature or gain.
 2. Aradiometer system according to claim 1, wherein a quadrature hybrid andamplifier circuit comprises a MMIC chip.
 3. A radiometer systemaccording to claim 1, wherein a quadrature hybrid and amplifier circuitcomprises discrete printed hybrids and low noise amplifiers.
 4. Aradiometer system according to claim 1, wherein said RF board is formedfrom a soft board or ceramic material.
 5. A radiometer system accordingto claim 1, wherein said radiometer system is sized and of such weightto fit directly into an antenna focal plane.
 6. A radiometer systemaccording to claim 1, wherein said housing further comprises a coverhaving RF launch openings.
 7. A radiometer system according to claim 6,wherein said RF launch openings are sized to allow for one wavelengthspacing between any sensing radiators.
 8. A radiometer system accordingto claim 1, and further comprising a video signal chopper amplifiercircuit on said controller board that receives said detected signal andaids in eliminating bias and reducing 1/f noise.
 9. A radiometer systemaccording to claim 1, wherein said detection circuit comprises at leastone pair of balanced diodes.
 10. A radiometer system according to claim1, wherein said controller board is adapted to interface with anexternal display system.
 11. A radiometer system according to claim 1,and further comprising a plurality of sensing radiators on said RFboard.
 12. A radiometer system according to claim 1, and furthercomprising an analog-to-digital conversion circuits on said controllerboard for digitizing detected signals received from said integrationcircuit for processing by said microcontroller.
 13. A radiometer systemaccording to claim 1, and further comprising isolation vias on said RFboard that isolate the quadrature hybrid and amplifier circuits.
 14. Aradiometer system comprising: a housing; an RF board contained withinthe housing and including a plurality of balanced radiometer channelsthat receive an unknown signal and known reference, each balancedradiometer channel having at least one quadrature hybrid and amplifiercircuit to provide distributed gain and an amplified output signal and adetection circuit that receives and detects the amplified output signalforming a detected signal; and a controller board contained within thehousing and having a integration circuit and microcontroller thatreceives the detected signal and performs video signal digitization andconditioning and real-time corrections on any radiometer channels toaccount for changes in temperature or gain, and provides a sensitivityof less than about 1° k and has a weight less than about three pounds,and is sized to fit directly into an antenna focal plane.
 15. Aradiometer system according to claim 14, wherein a quadrature hybrid andamplifier circuit comprises a MMIC chip.
 16. A radiometer systemaccording to claim 14, wherein a quadrature hybrid and amplifier circuitcomprises discrete printed hybrids and low noise amplifiers.
 17. Aradiometer system according to claim 14, wherein said RF board is formedfrom a soft board or ceramic material.
 18. A radiometer system accordingto claim 14, wherein said housing further comprises a cover having RFlaunch openings.
 19. A radiometer system according to claim 18, whereinsaid RF launch openings are sized to allow for one wavelength spacingbetween any sensing radiators.
 20. A radiometer system according toclaim 14, and further comprising a video signal chopper amplifiercircuit on said controller board that receives said detected signal andaids in eliminating bias and reducing 1/f noise.
 21. A radiometer systemaccording to claim 14, wherein said detection circuit comprises at leastone pair of balanced diodes.
 22. A radiometer system according to claim14, wherein said controller board is adapted to interface with anexternal display system.
 23. A radiometer system according to claim 14,and further comprising a plurality of sensing radiators on said RFboard.
 24. A radiometer system according to claim 14, and furthercomprising an analog-to-digital conversion circuits on said controllerboard for digitizing detected signals received from said integrationcircuit for processing by said microcontroller.
 25. A radiometer systemaccording to claim 14, and further comprising isolation vias on said RFboard that isolate the quadrature hybrid and amplifier circuits.
 26. Aradiometer device comprising: a substrate board, a plurality of balancedradiometer channels formed on the substrate board that receive anunknown signal and known reference, each balanced radiometer channelcomprising: at least one quadrature hybrid; and an amplifier circuit toprovide distributed gain and an amplified output signal, and a detectioncircuit that receives and detects the amplified output signal forming adetected signal to be output to an integration circuit andmicrocontroller that receives the detected signal and performs videosignal digitization and conditioning and real-time corrections on anyradiometer channels to account for changes in temperature or gain.
 27. Aradiometer device according to claim 26, wherein a quadrature hybrid andamplifier circuit comprises a MMIC chip.
 28. A radiometer deviceaccording to claim 26, wherein a quadrature hybrid and amplifier circuitcomprises discrete printed hybrids and low noise amplifiers.
 29. Aradiometer device according to claim 26, wherein said substrate board isformed from a soft board or ceramic material.
 30. A radiometer deviceaccording to claim 26, wherein said detection circuit comprises at leastone pair of balanced diodes.
 31. A radiometer device according to claim26, and further comprising a plurality of sensing radiators on saidsubstrate board.
 32. A radiometer device according to claim 26, andfurther comprising isolation vias on said substrate board that isolatethe quadrature hybrid and amplifier circuits.